Complementary metal-oxide-semiconductor (CMOS) technology is the dominant semiconductor technology used for the manufacture of ultra-large scale integrated (ULSI) circuits today. Size reduction of the metal-oxide-semiconductor field-effect transistor (MOSFET) has provided significant improvement in the speed performance, circuit density, and cost per unit function of semiconductor chips over the past few decades. Significant challenges are faced when CMOS devices are scaled into the sub-100 nm regime. An attractive approach for additional improvement of CMOS transistor performance exploits strain-induced band-structure modification and mobility enhancement to increase the transistor drive current. Enhanced electron and hole mobilities in silicon (Si) under biaxial tensile strain can be achieved. Enhanced electron and hole mobilities improve the drive currents of N-channel and P-channel MOSFETs, respectively.
As shown in FIG. 1a, many designs of strained silicon layers for transistor fabrication utilize thick buffer layers or complex multi-layer structures on a bulk silicon substrate 114. The conventional strained Si substrate technology utilizes a thick silicon-germanium (SiGe) graded buffer layer 110 with thickness in the order of micrometers. Formation of such a thick graded buffer layer 110 typically requires several tens of minutes to several hours and is an expensive process. A relaxed SiGe layer 112 overlies the graded buffer layer 110. The relaxed SiGe layer 112 has a larger natural lattice constant than that of silicon. Relaxed crystalline silicon is said to be lattice-mismatched with respect to relaxed crystalline SiGe due to the difference in their lattice constants. As a result, a thin layer of silicon 116 that is epitaxially grown on the relaxed SiGe layer 112 will be under biaxial tensile strain because the lattice of the thin layer of silicon 116 is forced to align to the lattice of the relaxed crystalline SiGe layer 112 as shown in FIGS. 1b and 1c. A transistor 118 is formed in the silicon layer 116 and 118 includes a source 120, a drain 122 and a gate 124. Transistors fabricated on the strained silicon layer 116 will have enhanced electrical performance.
However, such a substrate might not be easily or economically introduced into a conventional CMOS process. Firstly, the growth of a thick graded SiGe buffer is an expensive and time-consuming process. Secondly, the thick graded buffer layer introduces a lattice mismatch with the underlying substrate, resulting in a disperse, three-dimensional misfit dislocation network. Strain-relieving glide of threading dislocations is facilitated. Dislocations formed in the graded buffer can propagate to the wafer surface, resulting in a defect density in the order of 104-105 defects per square centimeter. Such a high defect density presents a significant barrier for the production of integrated circuits using such substrates.
Thirdly, the underlying strain fields of the misfit arrays result in a characteristic cross-hatch surface roughness. This surface roughness can be a significant problem as it potentially degrades channel mobility in active devices. In addition, conventional CMOS processes utilize high processing temperatures, especially during the formation of isolation structures. Isolation structures such as shallow trench isolation (STI), local oxidation of silicon (LOCOS), and their variants are in widespread use on bulk substrates today. High temperatures favor the formation of dislocations and increase the defect density. In fact, the defect density in strained silicon substrates has been observed to increase with prolonged annealing at high temperatures. The formation of isolation structures in substrates with a SiGe layer is also challenging as oxides formed on SiGe typically have a high interface state density.
Recently, T. A. Langdo et al., in a paper entitled “Preparation of novel SiGe-free strained Si on insulator substrates,” published at the 2002 IEEE International SOI Conference, October 2002, pp. 211-212 reported a SiGe-free silicon-on-insulator (SOI) substrate where strained silicon is incorporated. In the work of T. A. Langdo et al., a compositionally graded SiGe layer was grown on a silicon substrate followed by the growth of a relaxed SiGe layer and a tensile-strained silicon layer to form a donor wafer. Hydrogen was implanted into the donor wafer to induce a cleave plane, and the donor wafer was bonded to an oxidized silicon target wafer. A first anneal effects a cleavage along the cleave plane and a second anneal increases the bond strength so that a strained-silicon-on-insulator wafer is formed.
This process suffers from several disadvantages. Firstly, the surface roughness of the strained silicon layer has a root-mean-square value of about five angstroms and may impact device characteristics as well as the strength of bonding between the donor wafer and the target wafer. The high surface roughness of the strained silicon layer is related to the cross-hatch roughness resulting from the growth of the graded SiGe buffer or a lattice-mismatched substrate. Secondly, the strained silicon layer still suffers from high defect density because it overlies a graded SiGe buffer layer where there is an abundance of upward propagating threading dislocations. Therefore, any defects in the strained silicon layer is also transferred to the final strained-silicon-on-insulator wafer.
In the strained-silicon-on-insulator wafer, the strained silicon layer is in direct contact with an insulator layer. By bonding the strained silicon on an insulator layer, the strain in the strained silicon layer is partially relaxed. In the abovementioned paper published by T. A. Langdo et al., the strained silicon layer in the strained-silicon-on-insulator wafer has a strain of about 1%. Based on the germanium content in the relaxed SiGe layer in the donor wafer, the strain in the strained silicon layer should have been about 1.2% if there is no strain relaxation. Therefore, only 83.3% of the maximum possible strain is retained in the strained silicon layer in the final strained-silicon-on-insulator wafer.